Cyclotrons are circular particle accelerators, which are used to accelerate positive or negative ions up to energies of a few MeV or more. This type of equipment is employed in various fields such as industry or medicine, more precisely in radiotherapy for the production of radioisotopes, or in proton therapy with a view to treating cancer tumors.
Cyclotrons generally comprise five main components: the ion source which generates the ionized particles, the device for vacuum confinement of the ionized particles, the electromagnet which produces the magnetic field that guides the ionized particles, the high-frequency accelerator system intended to accelerate the ionized particles, and the extraction device making it possible to deviate the ionized particles from their acceleration trajectory then remove them from the cyclotron in the form of a beam with a high kinetic energy. This beam is then directed at the target volume.
In the ion source of a cyclotron, the ions are obtained by ionizing a gas medium consisting of one or more gases in a closed compartment, by means of electrons accelerated strongly by cyclotron electron resonance under the effect of a high-frequency magnetic field injected into the compartment.
Such cyclotrons can be used in proton therapy. Proton therapy is intended to deliver a high dose in a well-defined target volume to be treated, while sparing the healthy tissue surrounding the volume in question. Compared with conventional radiotherapy (X-rays), protons have the advantage of delivering their dose at a precise depth which depends on the energy (Bragg peak). Several techniques for dispensing the dose in the target volume are known.
The technique developed by Pedroni and described in “The 200-MeV proton therapy project at the Paul Scherrer Institute: conceptual design and practical realization” MEDICAL PHYSICS, January 1995, USA, vol. 22, No. 1, pages 37-53, XP000505145 ISSN: 0094-2405, consists in dividing the target volume into elementary volumes known as “voxels”. The beam is directed at a first voxel and, when the prescribed dose is reached, the irradiation is stopped by abruptly deviating the beam by means of a fast-kicking magnet. A scanning magnet is then controlled so as to direct the beam at a next voxel, and the beam is reintroduced so as to irradiate this next voxel. This process is repeated until all of the target volume has been irradiated. One of the drawbacks of this method is that the treatment time is long because of the successive stops and restarts of the beam between two voxels, and may be as much as several minutes, in typical applications.
Patent application WO00/40064 by the Applicant describes an improved technique, referred to as “pencil beam scanning”, in which the beam does not have to be stopped between the irradiation of each individual voxel. The method described in this document consists in moving the beam continuously so as to “paint” the target volume layer by layer.
By simultaneously moving the beam and varying the intensity of this beam, the dose to be delivered to the target volume can be configured precisely. The intensity of the proton beam is regulated indirectly by altering the supply current of the ion source. To this end, a regulator is employed which makes it possible to regulate the intensity of the proton beam. This regulation, however, is not optimal.
Another technique used in proton therapy is the technique referred to as “Double Scattering”. In this technique, the irradiation depth (i.e. the energy) is modulated with the aid of a wheel, referred to as a modulation wheel, rotating at a speed of the order 600 rpm. The absorbent parts of this modulator consist of an absorbent material, such as graphite or lexan. When these modulation wheels are manufactured, the depth modulation which is obtained is fairly close to predictions. The uniformity nevertheless remains outside the desired specifications. In order to achieve the specifications in respect of uniformity, rather than re-machining the modulation wheels it is less expensive to employ beam intensity regulation which is synchronized with the speed of rotation of the energy modulator. The modulation function is therefore established for each energy modulator, and is used as a trajectory which is provided as a setpoint to the beam intensity regulator. Rapid and accurate regulation of the intensity of the beam extracted from a particle accelerator is therefore also necessary in the double scattering techniques which use such a modulation wheel.